1. Field of the Invention
The present invention relates to an image forming apparatus which utilizes an electrophotographic process, such as an electrophotographic copier, an electrophotographic printer or the like.
2. Description of the Related Art
A conventional image forming appartus is configured, for example, as shown in FIG. 6. That is, a cylindrical electrophotographic photosensitive member (hereinafter termed a "photosensitive drum") 1, serving as an image bearing member, is provided within a main body 8 of the image forming apparatus, and rotates in the direction shown by arrow R1 around its shaft. The surface of the photosensitive drum 1 is uniformly charged by a charging roller 2, serving as a charging member, and then an electrostatic latent image is formed by an exposure device 3. A developing device 4 includes a hopper 4b for storing a toner 4a, and a developing roller 4c, serving as a developer carrying member. The developing roller 4c forms a visual image by supplying the electrostatic latent image formed on the photosensitive drum 1 with the toner 4a while regulating the toner 4a carried on the developing roller 4c by a developing blade 4d, serving as a developer regulating member, contacting the developing roller 4c.
The toner image on the photosensitive drum 1 visualized by the toner 4a in the developing device 4 in the above-described manner is transferred onto a transfer material 9 by a transfer roller 5, serving as a transfer member. The transfer material 9 is fed from a sheet feeding cassette 11 by a sheet feeding roller 10, and is sent to the transfer roller 5 while being synchronized with the toner image on the photosensitive drum 1 by registration rollers (not shown). The transfer material 9 having the toner image transferred thereto is then conveyed to a fixing device 6 so that the toner image is fixed by heat or pressure to form a recorded image. On the other hand, particles of the toner 4a remaining on the photosensitive drum 1 which have not been transferred by the transfer roller 5 are removed by an elastic cleaning blade 7a of a cleaning device 7 and are stored in a waste-toner receptacle 7b. Then, the surface of the photosensitive drum 1 is again charged by the charging device 2, and another image formation operation may be performed by repeating the above-described process.
Next, the configuration of the cleaning device 7 will be described in more detail with reference to FIG. 7.
In FIG. 7, the cleaning blade 7a comprises, for example, a chip-like rubber member 7a.sub.1 supported on a supporting plate 7a.sub.2, and removes toner particles remaining on the photosensitive drum 1 while being pressed against the photosensitive drum 1, which rotates in the direction shown by an arrow R1, with a predetermined pressure. A member 7c for preventing leakage of toner particles (hereinafter termed a "scraping sheet") is provided in contact with the photosensitive drum 1 at a portion upstream from the cleaning blade 7a in the direction of the rotation of the photosensitive drum 1, in order to prevent removed toner particles from dropping downward.
In the cleaning device 7 having the above-described configuration, when small-size collected substances, such as scraped powder by the cleaning blade 7a, toner particles and the like, are present at a portion where the edge portion of the cleaning blade 7a contacts the surface of the photosensitive drum 1, the effect of lubrication for the photosensitive layer of the photosensitive drum 1 by the collected substances is not experienced. Hence, the frictional force tends to be highest when starting use of the cleaning device 7. As a result, there arise the problems that, for example, the rotation torque increases, noise is generated, and the cleaning blade 7a is, in some cases, bent toward the downstream side in the direction of the rotation of the photosensitive drum 1. Conventionally, in order to prevent such a problem, a powder lubricant 7d is coated in advance on an edge portion of the cleaning blade 7a to reduce the frictional force when starting use of the cleaning device 7.
However, in the above-described conventional image forming apparatus, in general, as shown in FIG. 8, the contact region of the charging roller 2 (a region between A and A') is formed in the longitudinal direction of the photosensitive drum 1, the contact region of the transfer roller 5 (a region between B and B') is formed within the contact region of the charging roller 2, and the contact region of the cleaning blade 7a is formed so as to cover the entire region including the contact region of the charging roller 2.
By thus regulating the lengths of the charging roller 2, the transfer roller 5 and the cleaning blade 7a with respect to the photosensitive drum 1, the surface potential of the photosensitive drum 1 at the contact position of the cleaning blade 7a is distributed as shown in FIG. 8. That is, the region between A and A' where the charging roller 2 contacts the photosensitive drum 1 is uniformly charged to a negative potential by the charging roller 2, and a positive bias voltage is applied to the region between B and B' where the transfer roller 5 contacts the photosensitive drum 1.
Thus, in the distribution of the surface potential of the photosensitive drum 1 at the contact position of the cleaning blade 7a, regions between A and B and between B' and A' assume a negative potential because these regions are negatively charged by the charging roller 2 but are not positively charged by the transfer roller 5. On the other hand, the region between B and B' assumes a potential of substantially 0 V because this region is negatively charged by the charging roller 2 and also is positively charged by the transfer roller 5.
The surface potential of the photosensitive drum 1 at the contact position of the cleaning blade 7a also is substantially 0 V, although somewhat more so than the potential in the region between B and B'.
That is, in the regions between A and B and between B' and A', an electric field is generated between the cleaning blade 7a and the surface of the photosensitive drum 1. By the Coulomb force due to the electric field, the lubricant 7d on the cleaning blade 7a is peeled and moves toward the photosensitive drum 1. Accordingly, the frictional force with respect to the surface of the photosensitive drum 1 increases at portions of the cleaning blade 7a where the lubricant 7d is stripped off, and the cleaning blade 7a is bent at these portions.